The JWST Spectroscopic Properties of Galaxies at z = 9−14

Abstract We characterize the JWST spectra of 61 galaxies at z = 9−14, including 30 newly confirmed galaxies. We directly compare the z > 9 spectroscopic properties against 401 galaxies at 6 < z < 9, with the goal of identifying evolution in the star formation histories and interstellar medium. We measure rest-UV emission-line properties and UV continuum slopes, while also investigating the rest-optical emission lines for the subset of galaxies at 9.0 < z < 9.6. With these spectra, we constrain the stellar masses, specific star formation rates (sSFRs), dust attenuation, and the average metallicity and abundance pattern of z > 9 galaxies. Our dataset indicates that the emission lines undergo a marked change at z > 9, with extremely large C III ], H β , and H γ equivalent widths becoming 2 and 3× more common at z > 9 relative to 6 < z < 9. Using the spectra, we infer the distribution of star formation rates (SFRs) on short (SFR 3Myr ) and medium (SFR 3 −50Myr ) timescales, finding that rapid SFR upturns (large SFR 3Myr /SFR 3 −50Myr ratios) are significantly more likely among z > 9 galaxies. These results may reflect a larger dispersion in UV luminosity at fixed halo mass and larger baryon accretion rates at z > 9, although other physical effects may also contribute. We suggest that the shift in star formation conditions explains the prevalence of extreme nebular spectra that have been detected at z > 9, with hard ionizing sources and nitrogen enhancements becoming more typical at the highest redshifts. Finally, we identify five z > 9 spectroscopically confirmed galaxies with red UV colors ( β ≳ −1.5), revealing either a small population with moderate dust attenuation ( τ V = 0.23−0.35) or very high density nebular-dominated galaxies with hot stellar populations.